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Anisotropic attosecond charge carrier dynamics and layer decoupling in quasi-2D layered SnS2. Calley N. Eads1, Dmytro Bandak1, Mahesh R. Neupane2, ...
ARTICLE DOI: 10.1038/s41467-017-01522-3

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Anisotropic attosecond charge carrier dynamics and layer decoupling in quasi-2D layered SnS2

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Calley N. Eads1, Dmytro Bandak1, Mahesh R. Neupane2, Dennis Nordlund

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& Oliver L.A. Monti1,4

Strong quantum confinement effects lead to striking new physics in two-dimensional materials such as graphene or transition metal dichalcogenides. While spectroscopic fingerprints of such quantum confinement have been demonstrated widely, the consequences for carrier dynamics are at present less clear, particularly on ultrafast timescales. This is important for tailoring, probing, and understanding spin and electron dynamics in layered and two-dimensional materials even in cases where the desired bandgap engineering has been achieved. Here we show by means of core–hole clock spectroscopy that SnS2 exhibits spindependent attosecond charge delocalization times (τdeloc) for carriers confined within a layer, τdeloc < 400 as, whereas interlayer charge delocalization is dynamically quenched in excess of a factor of 10, τdeloc > 2.7 fs. These layer decoupling dynamics are a direct consequence of strongly anisotropic screening established within attoseconds, and demonstrate that important two-dimensional characteristics are also present in bulk crystals of van der Waalslayered materials, at least on ultrafast timescales.

1 Department of Chemistry and Biochemistry, University of Arizona, 1306 East University Boulevard, Tucson, AZ 85721, USA. 2 Sensors and Electron Devices Directorate, US Army Research Laboratory, Adelphi, MD 20783, USA. 3 SLAC National Accelerator Laboratory, Stanford Synchrotron Radiation Lightsource, 2575 Sand Hill Road, MS 99, Menlo Park, CA 94025, USA. 4 Department of Physics, University of Arizona, 1118 East Fourth Street, Tucson, AZ 85721, USA. Correspondence and requests for materials should be addressed to O.L.A.M. (email: [email protected])

NATURE COMMUNICATIONS | 8: 1369

| DOI: 10.1038/s41467-017-01522-3 | www.nature.com/naturecommunications

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ARTICLE

NATURE COMMUNICATIONS | DOI: 10.1038/s41467-017-01522-3

Results Mapping of the conduction band of SnS2. As will be discussed in more detail below, our experimental approach is based on resonant photoemission to probe the evolution of excited states on the timescale of a core–hole decay with sub-fs time resolution. In order to properly interpret the resonant photoemission results with respect to charge-carrier dynamics, we first need to understand the character of the excited electronic states reached in the initial X-ray absorption (XA) process, in particular the excitation of electrons into the SnS2 conduction band. In this study, we excite from Sn 3d levels (M-edge) to the SnS2 conduction band so as to access different orbital and spin characters in the conduction 2

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n recent years, the family of two-dimensional (2D)-layered transition metal dichalcogenides (TMDs) has gained significant interest due to their unique layer-dependent electronic properties, including layer-dependent band structures1, transition from indirect-to-direct bandgap2,3, and the ability to support substantial spin4,5 and valley6–8 polarization. Consequently, TMDs show promise as highly efficient field-effect transistors9,10, photovoltaics11 and spintronic devices12, and offer new avenues toward quantum computing. This promise hinges on a detailed understanding of the fundamental physics at play in TMDs, with particular emphasis on the spatially highly anisotropic electronic properties and the resulting consequences of quantum confinement in 2D. Indeed, exquisite band structure measurements using state-of-the-art angle-resolved photoemission (ARPES) have elucidated the electronic structure of TMDs13–15 and opened avenues toward tailoring bandgap and electronic properties, e.g., with different interlayer twist angles16. Despite this emerging body of ARPES work and while the electronic structure and excitations in TMDs have been investigated widely14,17,18, the extent to which the anisotropic electronic structure of layered materials confers 2D character already in the bulk crystals is not yet fully understood. If bulk crystals already exhibit essential aspects of 2D materials, simplification of device fabrication protocols may be anticipated, broadening their use in novel electronic devices. A number of studies have indeed suggested that layers in bulk TMDs, such as ReS219, WSe220, MoS221, or in graphite22, are sufficiently electronically decoupled to present as 2D materials, e.g., in terms of spin polarization. The observation of anisotropic screening and carrier dynamics in the time-domain would constitute a direct probe of layer decoupling in bulk crystals, but such studies are at present missing due to the extremely short timescales involved and the difficulty of spectroscopically resolving the anisotropic dynamical processes. Here we address this open question by investigating the ultrafast carrier dynamics in the layered semiconductor SnS223–25. Using core–hole clock spectroscopy26, we observe spin-dependent anisotropic charge transfer on attosecond timescales in quasi-2D bulk SnS2, and show that already in bulk crystals individual layers are indeed strongly decoupled and essentially 2D. Past ultrafast spectroscopic studies have primarily focused on excitonic carrierrelaxation dynamics in bulk TMDs27,28 with timescales of 3.0

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hν= 498 eV

Sn M4

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